The effects of a tumor burden on the natural and acquired resistance of mice to experimentally induced infections is being evaluated. The athymic mouse (nu-nu) and the immunocompentent hetozygote (mu plus) littermate have been selected as hosts. The tumor chosen for initial experiments is the methlcolanthrene induced Meth A/B fibrosarcoma: it can be considered syngeneic since the derivation of the tumor and the genetic background of the athymic mouse colony is Balb/c. Resistance to Listeria monocytogenes, Salmonella typhimurium, Streptococcus pneumoniae, Herpes simplex type II virus and Ectromelia virus is being evaluated in tumor bearing mice. To date we have established the kinetics of tumor growth in the athymic mice and immunocompetent animals and have collected base-line data to quantify resistance of these mice to the agents enumerated above. In addition we have accessed the burden of an established subcutaneously implanted tumor on resistance of the immunocompetent mice to infection. Data from a limited number of experiments indicate that tumor growth results in an increased resistance to L. monocytogenes infection. This effect was not observed with other bacterial or viral agents. Studies are in progress which will attempt to establish the temporal relationship between tumor growth and the aquisition of increased resistance to listeria.